do not delete executables of failed unittest

Make deletes targets that failed to build by default.
It's kind of annoying for unittests because I have to manipulate the Makefile to get the command and rerun the `unittest` target before I have an executable to debug.
On the downside this might sometimes preserve corrupt executable files when dmd or the linker crash.
An alternative option would be to use a [`QUIET`](https://github.com/D-Programming-Language/dmd/blob/master/test/Makefile#L85) variable.